Phase control is one commonly employed method for controlling the voltage applied to a motor in a power tool. Motor operation is controlled by switching the motor current on and off at periodic intervals that are synchronized with the alternating current (AC) input signal. The switching operation is achieved through the use of an electronic switch, such as a triac, coupled in series with the motor. One potential concern for phase-controlled power tools is that the triac can fail. Failure of the triac can be melting, fusing, or cessation of commutating off, either permanently or temporarily. This concern is relatively small for power tools having mechanical power switches that enable the tool operator to turn off the motor even if the triac were to fail. However, this concern poses a greater concern as tool manufacturers look to replace the power switches with switches that do not conduct the power being delivered to the motor.
Many power tools also implement a “no-volt release” feature. Briefly, a no-volt release feature prevents a power tool from operating when it is plugged into an AC power outlet when the power switch is in an ON position (i.e., closed). In a typical implementation, the no-volt release feature will not prevent the tool from operating if the triac short circuits, thereby causing inadvertent tool operation. Therefore, it is desirable to provide a protective redundant subsystem that monitors the switching operation of a triac in a power tool application.
This section provides background information related to the present disclosure which is not necessarily prior art.